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Asked: 2026-05-23T12:21:39+00:00

While reading Joe Albahari’s excellent book Threading in C# I came across the following

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While reading Joe Albahari’s excellent book “Threading in C#” I came across the following ambiguous sentence:

A thread-safe type does not necessarily make the program using it thread-safe, and often the work involved in the latter makes the former redundant.

(You can find the sentence on this page; just search for “indeterminacy” to quickly jump to the appropriate section.)

I am looking to use a ConcurrentDictionary to implement certain thread-safe data structures. Is the paragraph telling me that ConcurrentDictionary does not guarantee thread-safe writes to my data structure? Can someone please provide a counter-example that shows a thread-safe type actually failing to provide thread safety?

Thanks in advance for your help.

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1 Answer

  1. Editorial Team

    At the simplest, a thread safe list or dictionary is a good example; having each individual operation thread safe isn’t always enough – for example, “check if the list is empty; if it is, add an item” – even if all thread-safe, you can’t do:

    if(list.Count == 0) list.Add(foo);

    as it could change between the two. You need to synchronize the test and the change.

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